1. Field of the Invention
The present invention relates to an ultrasonic cleaning apparatus. More specifically, the present invention relates to an ultrasonic cleaning apparatus for cleaning a workpiece, such as a glass substrate for a LCD by the agency of ultrasonic waves.
2. Description of the Related Art
A cleaning method which immerses workpieces (objects to be treated), such as semiconductor wafers or a glass substrates for LCDs, sequentially in cleaning liquids, such as chemical solutions and rinsing liquids, contained in cleaning tanks is employed prevalently in a semiconductor device fabricating process.
An ultrasonic cleaning apparatus is employed in carrying out such a cleaning method. As shown in FIG. 12, a conventional ultrasonic cleaning apparatus comprises a cleaning tank 201 containing a cleaning liquid, such as a chemical solution 200, in which workpieces, such as semiconductor wafers W are immersed, an intermediate tank 203 containing pure water 202 in which a lower part of the cleaning tank 201 is immersed, and an ultrasonic generator 206 disposed at the bottom of the intermediate tank 203 and having a vibrating plate 205 and transducers 204 attached to the lower surface of the vibrating plate 205. The cleaning tank 201 comprises an inner tank 207 containing the chemical liquid 200 for actual cleaning of wafers W, and an outer tank 208 surrounding an open upper end part of the inner tank 207 to contain the chemical liquid 200 overflowed the inner tank 207.
Wafers W held in a wafer guide 209 are immersed in the chemical liquid 200 contained in the inner tank 207. Ultrasonic vibrations generated by the ultrasonic generator 206 are transmitted by the pure water 202 contained in the intermediate tank 203 to the chemical liquid 200 to shake particles and the like adhering to the wafers W off the wafers W by the ultrasonic vibrations.
If bubbles generated by the vibration of the transducers 204 adhere to the lower surface of the cleaning tank 201, ultrasonic wavers are reflected. Consequently, the efficiency of propagation of the vibrations to the chemical liquid 200 contained in the cleaning tank 201 is lowered, which may possibly reduce the cleaning ability of the ultrasonic cleaning apparatus. In the conventional ultrasonic cleaning apparatus, a nozzle 210 called a bubble eraser is disposed in the intermediate tank 203 and the pure water 202 is supplied through the nozzle 210 into the intermediate tank 203 so that the pure water 202 will overflow the intermediate tank 203 to remove bubbles from the lower surface of the cleaning tank 201.
If the pure water 202 starts overflowing the intermediate tank 203 at a part of its brim, the pure water 202 will not overflow the intermediate tank 203 at other parts of the brim of the intermediate tank 203 owing to the viscosity and surface tension of the pure water 202. Consequently, local currents of the pure water 202 are produced in the intermediate tank 203 and the pure water 202 stagnates in some parts in the intermediate tank 203, so that the bubbles adhering to the lower surface of the cleaning tank 210 cannot satisfactorily be removed.
Such a problem may be solved by inclining the lower surface of the cleaning tank 201 or by supplying pure water at an increased rate into the intermediate tank 203. However, there is a limit to the inclination of the lower surface of the cleaning tank 201 and the lower surface of the cleaning tank 201 cannot be inclined at an inclination sufficient to remove bubbles satisfactorily. Although bubbles can be removed to some extent by increasing the flow rate of the pure water, bubbles cannot completely be removed by such a means. Furthermore, increase in the flow rate of the pure water 202 entails disadvantageously increase in the amount of waste water.